A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material, such as methanol, ethanol, or natural gas. Such a fuel cell is a clean energy source that may replace fossil fuels. It includes a stack composed of unit cells, and produces various ranges of power. Since it has four to ten times higher energy density than a small lithium battery, it has been highlighted as a small portable power source.
Power generation by fuel cells is based on an oxidation reduction reaction generated on electrodes. Highly active catalysts are essential to improve the performance of the fuel cell. Now, the most popular material for the catalyst contains platinum (Pt), and with the content ratio of Pt increasing, the cost of the catalyst also increases. Therefore, there is a need to increase the efficacy of the catalyst and to produce a stable catalyst for the catalytic reaction.
Carrier materials, also refer to support, of the catalysts affect the metal particle sizes, electrochemistry and catalytic reaction rate of the catalysts. Many researchers focus on the carrier materials of the catalysts. An electro-catalyst is needed to induce the desired electrochemical reactions at the electrodes or, more precisely, at the electrode-electrolyte interfaces. The electro-catalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum supported on carbon. Most popular carrier materials for the catalyst are carbon materials. The characteristics for an ideal carrier comprise: good electron transfer ability or proton transfer ability, increasing transfer effect of the electrons or protons, large surface for metal particles acting uniformly, showing excellent metal particle absorption ability, stable catalytic ability without being affected by current, chemical stability, porous structure for fuel transfer, cost, etc. As for carbon fibers, a vapor-grown carbon fiber, a carbon nanotube and a PAN type carbon fiber are known. However, in any of the reports which have been made public to date, a technique to produce an electrode comprising a carbon fiber on which fine catalyst particles are uniformly carried with a high density has not been described.
Except carbon materials for the carrier of the catalysts, many metal oxides are also used as the carriers, such as Al, Si, Sn, Ti, Ni, Zr, La and Ce. Electric conductivity of such carriers is worse than carbonfibers, but they show good metal absorption ability so that metal particles separate on the surface of the carriers uniformly. Further, metal particles will not be lost after long term use. Still further, the carriers containing a Ru element could provide active oxygen molecules to oxidize Pt—CO to increase the anti-toxin ability of the electrodes.
As described in Jiun-Ming Chen et al., a gel precursor, made by TiO2, Pt and Ru which were coated on carbon fibers for redox reactions, and the diameters of the above metal particles were reduced 1 to 2 nm when comparing with the catalyst without adding TiO2, and the alloy condition described in Jiun-Ming et al. performed better than conventional ones. Now, it is known that TiO2 increases the well-separation between the catalyst particles.
Huanqiao Song et al. disclosed that TiO2 was applied on the surface of a CNT (carbon-nanotube) for carrying nano metal particles, such as Pt, of the catalyst in ethanol fuel cells. When the ratio of CNT:TiO2 equaled to 1:1, the ethanol fuel cells showed the best performance. When the content of CNT was increased, the electric conductivity reduced and the CO desorption ability also reduced. TiO2-CNT complex carriers show excellent CO stripping ability. Huanqiao Song et al. also used TiO2 nanotube as the carriers. The performance of TiNT carriers showed more excellent CO stripping ability than CNT or TiO2 particles. In CO stripping experiments, catalysts containing TNT/Pt/C show the lowest CO stripping electric potentials. Further, different calcination temperatures show different water content and also affect CO stripping ability. When water content in the carriers increased, the electric potentials for CO stripping would be reduced. However, when the calcination temperature was higher than 400° C., the water content in the carrier could not be self-supplemented.
When metal oxides were used as carriers, they provided bi-functional effects including increasing the anti-toxin ability of carbon monoxide, and metal oxides carriers also increase the absorption ability of PtRu alloy and also produce smaller particles. However, electric conductivity was worse. Therefore, there is a need to increase the electric conductivity of the metal oxides carriers to produce electrons from the surface of metal particles smoothly and prevent catalytic efficacy reduction.